Competitive effects of overtopping vegetation on Douglas-fir morphology in the Oregon Coast Range

Abstract

The response of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) saplings to various levels of brush overtopping when growing on three north-facing sites in the Oregon Coast Range was investigated for two consecutive years. A fisheye (hemispherical) photographic technique combined with a digitizing computer system was used to determine the percentage of visible skylight unimpeded by an overtopping brush canopy. Douglas-fir saplings were generally smaller in size under increasing levels of overtopping plant competition. The size of the basal stem diameter was most negatively affected and therefore the strongest indicator of possible competition from overtopping vegetation. A similar but weaker relationship was found for tree height. Subsequent-year (N+l) sapling height and basal stem diameter were strongly determined by the previous year's (N) sapling size and the degree of overtopping. Similar but more moderate relationships were found for predicting potential leader growth in Year N+1. Although strong empirical relationships were found, the exact mechanisms of competition were not identified. These results suggest that overtopping vegetation has both a negative impact on the current Douglas-fir size and a compounding negative effect on growth in the subsequent years. The results further demonstrate the importance of controlling competition from overtopping brush in the early years of reforestation. Management and research implications of this study are discussed.